A storage battery that, when power is excessively generated, stores electricity and compensates for power shortage with the electricity is representative of the techniques to smooth the output of power generation utilizing irregularly fluctuating and unstable energy such as renewable energy. Sodium-sulfur batteries, redox flow batteries, lithium batteries, lead batteries and the like are known as examples of large-capacity storage batteries. These batteries are all chemical secondary batteries, and can output the stored energy only in the form of electricity.
In addition, the technology referred to as compressed air energy storage (CAES) configured to store, in place of electricity, compressed air discharged from a compressor when power is excessively generated and to reconvert the compressed air to electricity by an air turbine power generator or the like when needed is known as another equipment for smoothing (Patent Document 1).